The correct answer is C. Longitude of a place is the angular distance between the meridian of the place and the Prime Meridian, which is located at Greenwich, England. The Prime Meridian is the line of longitude that divides the Earth into the Eastern and Western Hemispheres. It is also the zero-degree line of longitude, and all other lines of longitude are measured east and west of it. The International Date Line is an imaginary line that runs from the North Pole to the South Pole and separates two consecutive calendar dates. It is located about 180 degrees east and west of the Prime Meridian.
Option A is incorrect because the standard meridian is the meridian that is used as the basis for timekeeping in a particular area. The standard meridian for the United States is the 75th meridian west of Greenwich.
Option B is incorrect because the International Date Line is an imaginary line that runs from the North Pole to the South Pole and separates two consecutive calendar dates. It is located about 180 degrees east and west of the Prime Meridian.
Option D is incorrect because the longitude of a place is the angular distance between the meridian of the place and the Prime Meridian, not both the standard meridian and the Prime Meridian.